The present invention relates to absorbance detectors for high-performance liquid chromatography and, more particularly, to an absorbance detector for analyzing qualitatively a peak purity of a chromatogram by scanning two wavelengths alternately to calculate a ratio of absorption constants measured at these two wavelengths.
An absorbance detector using a spectroscope scanning at two wavelengths is well known. This type of detector drives a spectroscope scan at two wavelengths .lambda..sub.1 and .lambda..sub.2 to measure absorption constants at these wavelengths. In one prior-art technique, the absorption constant newly measured at one wavelength is combined with the most recently measured absorption constant at the other wavelength to form a ratio. With data measured alternately at wavelength .lambda..sub.1 at times (n-2) and (n) and at wavelength .lambda..sub.2 at times (n-1) and (n), two succeeding chromatograph ratios A.lambda..sub.1 (n-2)/A.lambda..sub.2 (n-1), A.lambda..sub.1 (n)/A.lambda..sub.2 (n+1) are formed. It will be noted that the calculated chromatography ratios use absorption constants measured at different points in time. That is, the first chromatograph ratio above uses absorption data taken at wavelength .lambda..sub.1 at time (n-2) while the absorption data taken at wavelength .lambda..sub.2 is taken at time (n-1), one time period removed. The second chromatograph ratio similarly uses data taken at times that are offset by one time unit. The lack of simultaneity in the measurements at the two wavelengths produces an error when the concentration of material being measured is changing. This follows from the fact that a chromatography ratio, for a single component, is constant if it is calculated for a single peak. However, if the data forming the ratio of absorption constants are not taken simultaneously, the chromotography ratio is not constant, even when measuring a single component, and therefore it is not possible to measure peak purity accurately.
Another absorbance detector of the prior art uses the ratio of moving averages of absorption constants measured at wavelengths .lambda..sub.1 and .lambda..sub.2. The chromatography ratio using a moving average is given by: EQU (A.lambda..sub.1 (n-2)+A.lambda..sub.1 (n)+. . . )/(A.lambda..sub.2 (n-1)+A.lambda..sub.2 (n+1)+. . . ).
Calculating a moving average uses the ratio of past data. This causes a time delay in determining the peak purity and therefore is not preferably in a minute-to-minute measurement system.